Flexible electronics made with organic semiconductors fit well on the surface of the human body or inside the body, because of the softness of the materials. Therefore, as a means of directly obtaining the biological information from cells or tissues, the use of flexible electronics has recently been attracting attention.
Temperature sensors have been attracting attention as one of their applications. Temperature sensors have been often used as switching elements for machine control and as thermal fuses. With the miniaturization and increasing complexity of the elements in recent years, there is a demand for temperature sensors that are more flexible and capable of finely adjusting the set temperature. Realization of a flexible temperature sensor capable of adjusting the set temperature close to the body temperature has been desired, since highly accurate measurements with little burden on the human body become possible in the medical field.
For example, as a temperature sensor, a temperature sensor having a characteristic in which the electric resistance value rapidly increases with a positive temperature coefficient (PTC) along with the temperature increase when reaching a certain temperature range (hereinafter, referred to as a “PTC characteristic”) has been proposed. For example, in Patent Documents 1 and 2, a resin composition having the PTC characteristic has been described. A resin composition for temperature sensors that has the PTC characteristic is capable of measuring the temperature changes through thermal expansion of a polymer containing conductive particles. The conductive particles are in contact with each other at low temperatures, and the resistance value between the electrodes is low. On the other hand, when the temperature increases and exceeds a certain temperature, the polymer is thermally expanded, and the gap is formed between the conductive particles to increase the resistance value. By measuring such changes in the resistance values, it is possible to measure the temperature change. In addition, it is also possible to carry out the control of the element in accordance with the temperature of the thermal fuse or the like by utilizing the characteristic in which the resistance value rapidly increases when the temperature reaches a certain temperature or higher.
Flexibility is required of such temperature sensors, and in order to achieve the flexibility, a thinner resin composition for temperature sensors that exhibits the PTC characteristic has been required. For example, in Patent Documents 3 and 4, it has been described that it is possible to realize a resin composition for temperature sensors that has the PTC characteristic with a thickness of 20 μm by means of printing or coating.
In order to obtain a temperature sensor with higher sensitivity, a large change in the resistance value at the temperature where the PTC characteristic is exhibited (hereinafter, referred to as a “PTC temperature”) has been required. If the change in the resistance value becomes larger, the output in response to the temperature change is increased, so as to constitute a temperature sensor with higher sensitivity. For example, in Patent Document 5, cocrystallization of two polymers for the purpose of increasing the change in the resistance value at the PTC temperature has been described.
In addition, in Patent Document 6, it has been described that by using a resin composition for temperature sensors that has a plurality of PTC temperatures, arbitrary PTC characteristics can be achieved in various temperature ranges.
Temperature sensors are required to exhibit cyclic repeatability of PTC characteristics in order to realize a stable measurement. In Non-Patent Document 1, a resin composition for temperature sensors with a PTC characteristic that does not change even when the temperature was changed repeatedly across the PTC temperature 100 times has been described.